PROS1 and infection: Protein NS4A was identified as the key protein responsible for the induction of membrane structures similar to the ones present at latter stages of infection, with a strict requirement for protease NS2B-3pro to cleave NS4A from NS4B at the 4A-2 K site in the NS4A-2K-NS4B polyprotein precursor44, possibly aided by a wedge insertion of helix α5 in one leaflet of the membrane32.